In general, an imaging apparatus using X-rays, ultrasound, or MRI (magnetic resonance imaging) is widely used in the fields of medical care and non-destructive inspection. On the other hand, studies on an optical imaging apparatus in which light emitted from a light source such as a laser or the like is propagated in an object such as a biological object or the like and information in the object is obtained by detecting the propagated light are actively conducted in the field of medical care. As one of the optical imaging technologies, photoacoustic tomography (PAT) is proposed.
In the technology of PAT, pulsed light generated from a light source is irradiated to an object, and an acoustic wave (hereinafter referred to as a photoacoustic wave) generated from a biological tissue that has absorbed energy of the light propagated/diffused in the object is detected at a plurality of positions. Subsequently, these signals are analyzed and information related to optical characteristic values of the internal portion of the object is visualized. With this operation, an optical characteristic value distribution in the internal portion of the object, especially a light energy absorption density distribution can be obtained.
An example of a detector for the acoustic wave includes a transducer using a piezoelectric phenomenon and a transducer using a change in capacitance and, in recent years, a detector using optical resonance is developed (Non Patent Literature 1: NPL 1). In addition, there is an example of a report on the detection of a sound pressure of ultrasound irradiated to a Fabry-Perot interferometer by using a CCD camera as a two-dimensional array sensor (Non Patent Literature 2: NPL 2).
FIG. 1 is a diagram of an acoustic wave detector using the optical resonance. As shown in the drawing, a structure in which light is resonated between reflection plates arranged in parallel with each other is called a Fabry-Perot interferometer. Hereinafter, the acoustic wave detector using the Fabry-Perot interferometer is referred to as a Fabry-Perot probe.
Such probe has a structure 103 in which a polymer film 104 having a thickness d is interposed between a first mirror 101 and a second mirror 102. Measurement light 105 is irradiated to the interferometer from the first mirror 101. At this point, a light amount Ir of reflection 106 is given by the following Expression (1):
                              [                      Math            .                                                  ⁢            1                    ]                ⁢                                                                                                I          r                =                                            4              ⁢              R              ⁢                                                          ⁢                              sin                2                            ⁢                              φ                2                                                                                      (                                      1                    -                    R                                    )                                2                            +                              4                ⁢                                                                  ⁢                R                ⁢                                                                  ⁢                                  sin                  2                                ⁢                                  φ                  2                                                              ⁢                      I            i                                              (        1        )                                φ        =                                            4              ⁢              π                                      λ              0                                ⁢          nd                                    (        2        )            Where Ii represents an incident light amount of the measurement light 105, R represents a reflectance of each of the first and second mirrors 101 and 102, λ0 represents a wavelength of the measurement light 104, d represents a distance between mirrors, and n represents a refractive index of the polymer film 104. φ corresponds to a phase difference when the light travels between the two mirrors, and is given by Expression (2).
An example of a graph obtained by graphing a reflectance Ir/Ii as the function of φ is shown in FIG. 2A. A periodic reduction in the reflected light amount Ir occurs, and the reflectance becomes lowest when φ=2 mπ (m is a natural number) is satisfied. When an acoustic wave 107 enters the Fabry-Perot probe, the distance between mirrors d changes. With this change, φ changes so that the reflectance Ir/Ii changes. By measuring a change in the reflected light amount Ir using a photodiode or the like, it is possible to detect the incident acoustic wave 107. As the change in the reflected light amount is larger, the intensity of the incident acoustic wave 107 is higher.
In order for the reflected light amount Ir to sharply change when the acoustic wave 107 enters, it is necessary to increase the change rate of the reflectance Ir/Ii with respect to the change in φ. In FIG. 2, the change rate thereof becomes largest at φm, i.e., the gradient becomes steep. Therefore, in the Fabry-Perot probe, it is preferable to perform the measurement after the phase difference is set to φm. By adjusting the wavelength λ0 of the incident light, it is possible to set the phase difference to φm.
A graph obtained by graphing the reflectance Ir/Ii as the function of λ0 is shown in FIG. 2B. Setting the wavelength to λm at which the change rate of the reflectance Ir/Ii is largest corresponds to setting the phase difference to φm, and the sensitivity thereby becomes maximum.
In this manner, in the Fabry-Perot probe, by adjusting the measurement wavelength λ0, it becomes possible to obtain high reception sensitivity by performing the measurement after setting the phase difference to φm.
In addition, in the Fabry-Perot probe, since the change in the reflected light amount only at a position to which the measurement light 105 is applied is measured, the spot region of the incident measurement light 105 becomes a region having the reception sensitivity. Consequently, by performing raster scanning with the measurement light using a galvanometer or the like, it is possible to obtain two-dimensional distribution data on the acoustic wave. By performing signal processing by using the obtained two-dimensional distribution data on the acoustic wave, an image is obtained.
On the other hand, by narrowing down the measurement light 105 using a lens or the like, it is possible to reduce the reception area. With this operation, the reception spot is reduced so that the resolution of the image at the time of reconstruction is improved. In addition, according to NPL 2, the Fabry-Perot probe has a wide reception frequency band of the acoustic wave. Because of the reasons described above, it becomes possible to obtain a minute image with high resolution by using the Fabry-Perot probe.